[3.x] Drag multiple resources onto exported array variable at once

[AaronRecord]

Backport of #37965

[AaronRecord]

It seems it's possible to drag non-valid resource types (e.g. dragging an audio file onto an array of textures) onto the array and it just creates a new empty element. It's not a big deal, but it seems there's code that's supposed to stop this from happening:
https://github.com/godotengine/godot/pull/50718/files#diff-17781aa265945f5f8988f4d66150a04b73fcdd99f5600b24409d731d24e6fbf1R442-R445, cc @EricEzaM.

It also prints an error message like so:

ERROR: Failed to set a resource of the type 'PackedScene' because this EditorResourcePicker only accepts 'Texture' and its derivatives.
   at: (editor\editor_resource_picker.cpp:753)

I'm not sure if this is a problem with the backport of if the original PR had this problem too.

[EricEzaM]

More issues caused by this damn code snippet:
#40383 (comment)

allowed_type was being set to subtype, but subtype is just Object in this case, not, for example, Texture. The string value "Texture" is stored in subtype_hint_string, so when the subtype is Object, we should also look to see if there is a subtype_hint_string, and make the allowed type that. I will submit a PR for this. 🙃

[AaronRecord]

#50805 should probably be merged first, then I can cherry-pick it as part of this PR, but other than that I think this PR is good.

[akien-mga]

#50805 should probably be merged first, then I can cherry-pick it as part of this PR, but other than that I think this PR is good.

It has now been merged :)

[AaronRecord]

I think this should be good to go now.

[akien-mga]

Thanks!